Many devices are known that can switch an electric current from a first to a second value in responset to some external stimulus. Such devices range from electromechanical relays to semiconductor devices (e.g., transistors, thyristors) and to superconducting Josephson junctions. Many such devices can be used to construct oscillators, which in turn can serve diverse functions, e.g., as sources of ac current or electromagnetic radiation, or as timing elements.
Despite the fact that many such current switching devices are already known to the art, there is continuing interest in new switching devices, especially devices based on novel principles and/or materials. As past experience shows, the discovery of a new device frequently results in the discovery of applications in which the new device has advantages over prior art devices, or in completely new applications. A simple but economically very significant example is the replacement of electromechanical relays with switching transistors in telephony. Based on such past experiences one can confidently assert that a novel currentswitching device is likely to lead to advances in technology and thus would be of considerable importance.
Until several years ago all known superconductors were elemental metals (e.g., Hg, the first known superconductor), alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, the superconductor with, unitl recently, the highest transition temperature T.sub.c). All these superconductors have relatively low resistivity in the non-superconducting state at temperatures relatively close to T.sub.c.
Several years ago it was discovered that some metal oxides can become superconductive, albeit at relatively low temperatures. Recently however, metal oxide superconductors were discovered that have a relatively high T.sub.c. See, for instance, J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B-Condensed Matter, Vol. 64, pp. 189-193 (1986); M. K. Wu et al, Physical Review Letters, Vol. 58(9), pp. 908-910; R. J. Cava et al, Physicaly Review Letters, Vol 58(9), pp. 1676-1679; and D. W. Murphy et al, Physical Review Letters, Vol. 58(18), pp. 1888-1890, the latter two incorporated herein by reference. The normal-state resistivity of these high T.sub.c oxide superconductors typically is relatively high compared to that of the prior art non-oxidic superconductors.
One aspect of the high T.sub.c superconductors that has prompted strenuous research efforts is their relatively low critical current density J.sub.c. For instance, in bulk samples of nominal composition Ba.sub.2 YCu.sub.3 O.sub.7, J.sub.c typically is below about 10.sup.3 A/cm.sup.2 at 77K, and efforts to produce bulk material having larger J.sub.c have not yet been successful. On the other hand, in thin films of the same nominal composition the research efforts have already had considerable success, and films with current densities of order 10.sup.5 A/cm.sup.2 at 77K and even higher have been produced. The significance of the fact that a major direction of the work in the high T.sub.c superconductive field is towards increasing J.sub.c will become apparent from the discussion below.